A second ATLAS II small animal PET scanner was completed during this reporting period. The first and second ATLAS systems differ from one another only in that the second system is based on Hamamatsu R7600 C-12 position-sensitive photomultiplier tubes (PSPMTs) whereas the first system is based on R7600 C-8 PSPMTs. This difference will allow ATLAS II to be easily and inexpensively upgraded to much higher spatial resolution by virtue of the much improved spatial linearity of the C-12 tube. ATLAS I, completed during the preceding reporting period, is now in use in the intramural research program to study organ function in small animals such as rats and genetically altered mice. Expanding collaborative studies between the Imaging Physics Laboratory and intramural investigators now include multi-modality imaging (PET, CT, OMRI and EPR) experiments in tumor-bearing mice, FDG and receptor imaging studies in the mouse and rat brain and studies of new positron-labeled pharmaceuticals created by the PET Department. During this reporting period work continued on enhancements and improvements to the ATLAS data acquisition software, user interface and applications program set including addition of new data acquisition protocols, a remote 3D OSEM supercomputer image reconstruction option and an extended image analysis and visualization package with multi-modality image registration capability. This latter addition is now being used to view spatially registered PET and CT scans of the same animal. These CT scans are obtained with a new small animal CT volume scanner installed during this reporting period immediately behind, and coaxial with, the ATLAS gantry. Registered CT/PET images will be used to correct the PET emission scans for radiation attenuation and to help identify structures labeled with the PET radiopharmaceutical.